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From: Richard Henderson <richard.henderson@linaro.org>
Message-ID: <32172d01-329a-764e-449c-efb83e353353@linaro.org>
Date: Wed, 17 Oct 2018 14:12:21 -0700
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Subject: Re: [Qemu-devel] [PATCH v6 11/14] target/arm: PMU: Add instruction
 and cycle events
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To: Aaron Lindsay <Aaron@os.amperecomputing.com>
Cc: Aaron Lindsay <aclindsa@gmail.com>, "qemu-arm@nongnu.org" <qemu-arm@nongnu.org>, Peter Maydell <peter.maydell@linaro.org>, Alistair Francis <alistair.francis@xilinx.com>, Wei Huang <wei@redhat.com>, Peter Crosthwaite <crosthwaite.peter@gmail.com>, Michael Spradling <mspradli@codeaurora.org>, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>, Digant Desai <digantd@codeaurora.org>

On 10/17/18 12:47 PM, Aaron Lindsay wrote:
> On Oct 16 17:04, Richard Henderson wrote:
>> On 10/10/18 1:37 PM, Aaron Lindsay wrote:
>>> + * Return the underlying cycle count for the PMU cycle counters. If we're in
>>> + * usermode, simply return 0.
>>> + */
>>> +static uint64_t cycles_get_count(CPUARMState *env)
>>> +{
>>> +#ifndef CONFIG_USER_ONLY
>>> +    return muldiv64(qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL),
>>> +                   ARM_CPU_FREQ, NANOSECONDS_PER_SECOND);
>>> +#else
>>> +    return 0;
>>> +#endif
>>> +}
>>
>> Usually we pass through the host cycle counter.
>> See cpu_get_host_ticks().
> 
> Why do you prefer cpu_get_host_ticks()? And are you suggesting this for
> just user-mode, or both system and user?

Just user-mode.  Providing a clock with unknown scaling is more useful than a
constant 0.


r~